Pulse keyed crystal controlled oscillator



Dec. 14, 1954 A. SZERLIP 2,697,172

PULSE KEYED CRYSTAL CONTROLLED OSCILLATOR Filed Feb. l5, 1954 ,Byz 7%4United States Patent O PULSE KEYED CRYSTAL CONTROLLED OSCILLATOR LosAngeles, Calif., assignor to Hughes Alexander Szerlip,

Culver City, Calif., a corporation Aircraft Company, of Delaware Thisinvention relates to a triggered crystal-controlled oscillator and moreparticularly to a crystal-controlled oscillator which oscillates onlyduring the duration of a pulse applied thereto.

Although the crystal-controlled oscillator disclosed herein can be usedin many circuits, it has particular utility in a precision markergenerator. The need for a precision marker generator to provide singleor multiple visual time base markers on a cathode ray display devicearises in connection with electronic systems such as radar,communication and laboratory instruments. This marker generator must behighly accurate and extremely stable in its operation.

In order to obtain the most precision, most precision marker generatorsemploy crystal-controlled continuous sine-wave oscillators. electronicsystem, since the timing of the markers generated during successivesweeps of the cathode ray display device will not coincide with thetiming of successive corresponding sweeps unless an exact harmonicrelationship exists between the oscillator frequency and the sweepfrequency. lt is therefore desirable to provide an oscillator which canbe trlggered into operation for a given time interval at the beginningof each sweep of the cathode ray display device so that the oscillationsalways bear the triggered requires shock excitation applied directly tothe quartz crystal. The output of this oscillator circuit consists ofdamped oscillations which last only one thousand microseconds. This istoo short a time base for many applications. ln addition, the decay inamplitude of the oscillations causes timing intervals errors.Furthermore, only special cuts of crystals can be shock excited intooscillation, and they oscillate in many modes.

lt is therefore an object of this invention to provide acrystal-controlled oscillator which is readily capable of beingtriggered into operation.

It is a further object of this invention to provide a crystal-controlledoscillator which sustains oscillations after being triggered intooperation.

It is a still further object of this invention to provide acrystal-controlled oscillator which sustains oscillations at a constantamplitude after being triggered into operation.

It is a still further object of this invention to provide acrystal-controlled oscillator which is triggered into operation by theleading edge of a square-wave pulse applied thereto to produceoscillations, having a constant amplitude, only for the duration of thepulse.

It is a still further object of this invention to provide acrystal-controlled oscillator readily capable of being triggered intooperation which employs standard crystal cuts.

Briefly, the invention contemplates the use of a bridge circuit, inwhich one arm includes a piezoelectric quartz crystal resonant at agiven frequency, its conjugate arm includes a tuned parallel LC circuitresonant at slightly higher than the given frequency, and the tworemaining arms include only resistances. Means are provided for shockexciting the tuned circuit into oscillation. The shock-excitedoscillations are amplified by a lirst amplifier and then applied to thebridge provide regenerative feedback to thereby sustain oscilla- Thislimits the flexibility of the circuit in proper phase to s tion. Themagnitude of the regenerative feedback is determined by the impedance ofthe bridge circuit, and this impedance in turn depends upon the crystalimpedance. Therefore, the oscillator will oscillate at that frequencywhere the impedance of the crystal is such as to provide the mostregenerative feedback. In order to provide greater frequency stabilitythe shock excited oscillations are separately amplified by a secondamplifier and then applied to the bridge circuit in proper phase toprovide degenerative feedback, the magnitude of which varies withcrystal impedance in an opposite sense with respect to the variation inthe regenerative feedback with crystal impedance. Therefore, at thefrequency at which there is the most regenerative feedback there is theleast degenerative feedback, and vice versa.

The features of the invention which are believed to be novel are setforth with particularity in the appended clairns.. The invention itself,however, both as to its orconjunction with the accompanying drawing, inwhich and schematic circuit diagram of the preferred embodiment of theinvention; and

Fig. 2 shows the waveforms of the input and output signals of theoscillator illustrated in Fig. 1.

Referring now to Fig. 1, the bridge circuit 10 comprises seriesconnected rst, second, third and fourth arms numbered 5, 6, 7 and 8,respectively. First arm 5 comprises a piezoelectric crystal 12 having aselected natural frequency. The second and fourth arms include resist-14 and 22, respectively. The third arm comprises a resistor 20 seriallyconnected to a parallel resonant circuit .t6-18 which is tuned to afrequency slightly higher than the natural frequency of the crystal.

One end of each of the second and fourth arms are connected respectivelyto the rst and second free ends of arm 5 to form junctions A and D,respectively. The third arm 8 is serially connected at one end to thefree end of the second arm 6 to form junction B and the other end ofthird arm 8 is serially of fourth arm 7 to form junction C.

Junction C of bridge circuit 10 is connected to a point of referencepotential. Electron tube 24 has its cathode 26 connected to junction Bof bridge circuit 10 and its anode 28 connected to a source of potentialwhich is positive relative to the point of reference potential.

A trigger pulse from an external source, such as, for example, themaster keyer in a radar system, is applied to the input of a negativegate generator 30. Negative gate generator 30 is any device, such as amonostable multivibrator, which produces a single negative squarewavepulse of a given duration which is initiated either in time coincidencewith or a given time delay after the application of a trigger pulsethereto. The output of negative gate generator 30 is applied to controlelectrode 32 of electron tube 24.

Electron tube 36 is utilized marker pulses are applied.

The output of electron tube 36 is also coupled through capacitors 50 and58 to the grid 60 of electron tube 56. Cathode 64 of electron tube 56 isconnected to the point of reference potential through bias resistance66. Anode 68 of electron tube 56 is connected to a source of positivepotential through load resistance 70. The anode 68 of electron tube 56is coupled to junction 0 A of bridge circuit 10 through capacitance 72.

connected to the free end Electron tube 74 has its control electroder'176 connected` to junction B ofbridge circuit tu, its cathode 7S-connected to junction D of bridge circuit 10, and its anode 80 connectedto the source of positive anode potential.

The ,met-lied; of; operation ofthe cicuitshown iny Fig, 1 willA now;beconsidered'., Electron` tube 214. is connected as ai cathode; followerwith; arm- 8410i bridge circuit it) being itsgcathode; loadv impedance,1n its quiescentI state electron tube, 2,4 draws. c nrrent throughinductance 1S andxresistancel... "Ehevcltagedrop acrossresistance 20properly biases electron, tube 24. Thel output impedance Qfi a.;Qathfle: followerk 24'. is extremely low because it` is normallyY in4 aiConducting condition. Therefore. when electrontube 214 is, iny aquiescent state. the impedance between' junctions B and C of bridgecircuit iti i's so low.; that there is no tendency. for the. circuit` tooscillate.

The; application of a; trigger pulse to negative gate generator 30results in a negative square-.wave pulse, showninliig. 2A, beingimpressed in t1 ,1,b e24.` The negative4 square-wavepulse has sufficientamplitudetocut off electron tube, 24 thereby greatly increasing theeffective impedanceexisting between junctions, B and4 C, ofbridge,circuit 10 and iny addition the sudden cuttingv olf of. electron tunedcircuit;composedv of capacitance lr6 andjinductance 18 being shockexcited into oscillation. rl`hey ensuing oscillation would graduallyvanish due to the damping of the circuit unless energy is continuouslyinjected back into` the oscillating circuit.

Cof', bridge circuitA 10, are appliedl through resistance 40tothe inputof amplifier tube 36. The purpose of tance- 40 isto provide sufficientbias for electron tube 36 to Overcome f the at junction B of bridgecircuit liu' due to the interelectrode, current ofjtube 2 4 when it isin its quiescent state.

The amplified'oscillations appearing in the output of electron tube 36are impressed onthe control electrode ofjarnpliiier tube 56'. The stillfurther amplified oscillations appearingat the anode of'electron tube 56are applied between junctionsV A and Cl of bridge circuit 10. T l'teVarms of bridge circuit 1t) betweenjunctions A and Bj'andbetweenjunctions B and'C form a voltage divider,

Theseoscillations, appearing between junctions B and' so that'a fixedproportion of theamplified'oscillations apj A and C ofbridge-circuit 10plied between junctionsv B and Cof bridgecircuit 1t) appear between;junctions 'and-arefedlback asV an inputl to electron tube 36. Since theoutputofj each of lelectron tubes 361and'56 is phase invertedl relativetov its respective input, they output of electron tubey 56, appearingbetween junctions A and C ofr bridge circuit 10; will bein phase withthe input to electron 36,` appearing between junctions B and C of bridgecircuit 10. Therefore, the ii'xed'proportionof the oscillations fed backto the-input ofeiectron tube 36 are of proper phase to provideregeneration, and the condifor-sustained oscillation are fulfilled.

Themanner by which crystallcontrols the frequency of oscillations willnow be discussed. It is well known that theequivalent circuit of acrystal is a serially connected capacitance, inductance and resistanceall` shunted bya capacitance. Such a circuit is series resonant at afrequency and is parallel resonant at a secondfrequency' slightly higherthan the first frequency. rIhe second frequency is utilized in thepresent invention. All other things being equal, the higher the totalimpedance presented by bridge circuit llt) between junctions A an C, thegreater will be the magnitude of the oscillations appearing betweenjunctions Al andl Cof bridge circuit 10. Furthermore, the higherv theimpedance of crystal 12; the higher-will bethe total impedance presentedby bridge circuit 10 between junctions A and C. Crystal lhas-its highestimpedance at the frequency at which it is parallel resonant, Therefore,the magnitude of the oscillations appearing between junctions A and ofbridge circuit 1t) will begreatest atvthe parallel resonant frequency ofcrystall 12;. Since afixed ,proportion of the oscillations appearingbetween junctions A andV C of bridge` circuitv 10 appear betweenjunctions B and C of bridge circuit 10, the magnitude of this` fixedproportion willi also be greatest at the parallel resonant frequencyof`crystal12. However,- the oscillations appearing between junctions Band C of bridge circuit 10 are applied as an-inputto electron tube 36,andl the magnitude of 'the-amplifiedvoscillations appearing betweenjunctions A and Cot bridge circuiti 101 is proportionalto. the, magthegrid of electron tube 24, results in the positive potentialwhich existsy first),

t nitude of the input applied to electron tube 36. Therefore, the effectis. cumulative and theoscillator will oscillate at a much higheramplitude at the parallel resonant frequency of crystal i2 than at anyother frequency.

Since the tuned circuit composed of capacitance lo and inductance 18 isresonant at a frequency somewhat higher than the natural resonant'frequency of crystal f2, that is, its` series resonant frequenc theoriginal shock excited oscillations will,v have a. frequencyapproximating the parallel resonant4 frequency of crystal 12, so thatthe oscillatorI will. loci; inv at theparallel resonant frequency of`crystal i2 after` no more thantwo cycles of oscillation. However, yifthe tuned circuit is resonant at a. frequency too far removed from theparallel resonant frequency of crystali l2; the crystal. will. not tairecontrol, and the oscillator will oscillate at a lower amplitude at theresonant frequency'v of= the tunedi circuit.

It has been found that with the circuit as so far described theoscillations include spurious frequencies. These spurious, frequenciescanbe removed and the frequency stability of` the oscillator improved byproviding a separate amplifier which produces a large amount ofdegenerative. feedback at frequencies other than the parallel' resonant`frequency of crystal 12 and a small amount of" degenerative feedback oreven additional regenerative feedback at the parallel .resonantfrequency of Bj of bridge circuit l0' and its cathode '73' connected tojunction D ofbridge circuit` liti. It will be seen that ifbridge-circuit l0 is balanced,` junctions B and D of bridge circuit 10will be at the same potential and electron tube 74' will-have no output.lf bridge circuit l@ is out of balance such that' junction D is. at ahigherl potential than junction B, electrontube 74 will have an outputwhich is out-'of phase-with the oscillations appearing betweenjunctionsA andC of bridgecircuit 10, and which has anamplitudeproportional` to the potential difference existingbetweenjunctions BandiDof'brid'ge circuit tu. Similarly, ifbridgecircuitlt) is :out of` balance such that junction B is at-a higherpotentialfthanjunction D, electron tube 74 will have an output., inphase with the oscillations appearing between junctions A- and C- andhavean arnpiitude proportionali to the potential difference existingbetween junctionsA B and' D. Crystal i2. and resistance 22 forma voltagedivider for the oscillations appearing betweenjunctions A and Cof bridgecircuit it). Since the impedance of crystal i2 is high at its parallelresonant frequencyqandlow at all other. frequencies, the potential atjunction at the parallel resonant'. frequency of crystal 12 than at anylother.l frequency. Thus, if theA parameters of the bridge circuitarefsuch: that it isl close to balance when crystal i2: is parallel.resonantv and has its highest irnpedance, thepotential atjunction D ofbridge circuit 10, when crystab 12, is= not parallel resonant andtherefore presents: a'relativelyylowimpedance, will be much higher thanthev potential at: junction B' of bridge circuit 10. Therefore, at theparallel resonant frequency of crystal i2; electron tube-74 Will havepractically no output, but at frequencies; other than the, parallelresonant frequency of crystal 12, electron tube I4 will'have a highoutput which isout of phase, with the oscillations applied betweenjunctions A and C of bridge circuit 10. Thus, electron tube 74 causesdiscrimination against spurious frequencies in favor of the parallelresonant frequency of crystal 12.

After the--endj of thenegative square-Wave pulse, electron tube24-again'draws current, and effectively shunts junctions B and Cofbridge circuit 10 with its low output impedance.- This causestheoscillator to immediately cease oscillating.

Theoutput of the. oscillatorshown in tained atthe output of electrontube 36 the input to utilization means 48.

While there has been described what is at present considered to beapreferred embodiment of the invention, it will be obvious to thoseskilled in the art that variouschanges and modifications may be madetherein without departing from the invention, and it is aimed in theappended claims to cover all such changes and modifications as fallwithin/the` true spirit and scope of the invention.

What is claimed is:v

ILAcrystaI-,controlledoscillator including a bridge Fig. 2B, is oband`applied as Drof bridge circuit 10; willy be relatively lower circuitcomposed of a nrst arm comprising a piezoelectric crystal resonant at agiven frequency, a second arm comprising a first resistance, one cid ofsaid second arm being connected to one end of said rirst arm at a rstjunction, a third arm comprising a capacitance and inductance saidtwo-stage ampliier.

A crystal-controlled oscillator a s set forth in claim 1, wherein saidmeans for shock exciting said tuned cirsad control electrode to saidsecond junction.

7. A crystal-controlled oscillator as set forth in claim 5, wherein saidthird junction is connected to a point of reference potential, andwherein said first amplifying means comprises a two-stage electron tubeamplifier having its input coupled to said second junction and itsoutput coupled to said first junction.

8. A crystal-controlled oscillator as set forth in claim 7, furtherincluding utilization means having its input to said second junction.

10. A crystal-controlled oscillator as set forth in claim 52 whereinsaid 11. A crystal-controlled oscillator as set forth inclaim 10,wherein said third arm of saidbridge circuit includes for applying saidsquare-wave pulse to said control electrode to effect the cut-off ofsaid electron tube only during said given duration.

12. A crystal-controlled oscillator as set forth in claim 1l, whereinsaid first amplifying means comprises a' twocoupled to said firstjunction.

13. A crystal-controlled oscillator as set forth in claim necting thecontrol electrode of said fourth electron tube to said second junction.

l5. A crystal-controlled oscillator as set forth in claim 14, furtherincluding utilization means having its input circuit coupled between theanode of said second electron tube and said point of referencepotential.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,303,862 Peterson Dec. 1, 1942 2,611,873 Gager et al Sept.23, 1952 2,638,548 MacNichol May 12, 1953

